The present invention relates to a method of producing a steel having high strength, high toughness and excellent weldability, by a combination of a specific condition of chemical composition of the steel and a specific condition for heating and rolling, as well as cooling after the rolling.
In recent years, the use of high tensile steel has become popular in the field of production of welded constructions in such as buildings, pressure vessles, ship building, line pipes and so forth, from the view point of economy and safety. This in turn gives rise to a demand for improved weldable high tensile steel. For attaining a higher safety and workability, the high tensile steel for welded constructions are required to have a high toughness, as well as superior weldability and weld zone characteristics. These requirements are becoming severer, year by year.
A controlled-rolling method (CR method) is widely used for the production of line pipe material, steel for low temperature use and so forth. Also, a so-called QT method in which quenching and tempering are effected subsequently to the rolling is known as a method which can cope with the above-stated demand. The CR method, however, has a practical limit in the increase of the strength, and encounters a deterioration in weldability and rise in costs when the amount of alloying addition is increased. The QT method is also disadvantageous in the cost of production of steel due to the necessity for the re-heating.
Under these circumstances, there is a vigorous movement for the development of a method called controlled-cooling method in which various measures are taken to save energy and natural resources, particularly alloying elements.
The steel produced in accordance with the controlled-cooling method have advantages of both of the CR method and the QT method. Namely, the steel produced by this method exhibits superior characteristics as a micro alloy steel or a steel having no special alloying element. Unfortunately, however, this steel had only a limited use and could not practically satisfy the strict demand for toughness in the base metal and weld zone as the materials for pipe lines and steels for low temperature use, because of the disadvantages or problems stated hereinbelow.
(1) The austenite grains become inconveniently coarser due to the excessively high heating temperature, resulting in a coarser microstructure after transformation through cooling and reduced low temperature toughness.
(2) Due to the low rolling reduction in the recrystallization zone and nonrecrystallized zone, the microstructure after transformation becomes coarse to reduce the low temperature toughness.
(3) Absorbed energy in the impact test is seriously lowered because of the two-phase region rolling which is conducted to improve the arresting characteristics for brittle fracture and to prevent softening due to welding. In consequence, the chance of brittle fracture initiation is increased and the resistance to the unstable ductile fracture is deteriorated.
(4) Martensite is formed if the cooling rate is too high, resulting in lower absorbed energy in the impact test. A tempering becomes inevitable to improve toughness.
(5) The microstructure and, hence, the hardness is not uniform in the through-thickness direction of the steel plate.
(6) Micro cracks are likely to be induced by H.sub.2 because of the water cooling effected immediately after the rolling.
(7) The toughness in the Heat-Affected Zone (HAZ) in weld is much inferior to that of the base metal, because no specific consideration is made as to the HAZ toughness.
Due to these problems or drawbacks, the steel produced by the controlled-cooling method has an extremely limited use.
Among the prior art methods of producing high tensile-strength low-alloy steel plates with good toughness, U.S. Pat. No. 4,184,898 developed by Ouchi et al is considered to be an invention which utilizes accelerated cooling subsequent to controlled heating and rolling.
Ouchi et al's. U.S. Pat. No. 4,184,898 is directed to obtain a steel having high strength and high toughness at low temperature but it does not positively aim at improving both weldability and the mechanical properties at the heat affected zone (HAZ) caused by welding.
On the other hand, the present invention is directed to the method of producing high tensile-strength low-alloy steel having superior weld zone properties.
So far as the chemical composition of the respective alloy is concerned there exists some extent of overlapping with respect to the allowable ranges of carbon, silicon, manganese, niobium and aluminum.
However, with respect to restriction to other chemical components the present invention differs from the U.S. Pat. No. 4,184,898 regarding critical limitation on the upper limits for sulphur, calcium, oxygen and nitrogen as well as specifically recited conditions concerning several ingredients represented by two formulas; ##EQU2##
As to thermal conditions and rolling, that is, heating, rolling reduction and cooling of the steel, the present invention differs greatly from the U.S. Pat. No. 4,184,898 particularly with respect to heating temperature, cooling speed and the temperature at which further cooling down to lower temperature has to be stopped.
Speaking of actual value of these thermal and rolling conditions, comparison will be made now between the present invention and the Ouchi, et al's. U.S. Pat. No. 4,184,898.
According to the present invention, the steel which satisfies the specified chemical restriction is heated at 900.degree.-1000.degree. C. and rolled to effect more than 60% of rolling reduction below 900.degree. C. and the rolling to be finished within a temperature range of between plus 20.degree. C. of Ar.sub.3 transformation temperature and minus 10.degree. C., then the rolled steel is cooled to 300.degree. C. or lower down to room temperature at a cooling rate of 15.degree.-60.degree. C./sec.
On the other hand, said patent to Ouchi, et al. comprises the steps of heating the steel at a temperature above plus 150.degree. C. of Ar.sub.3 transformation temperature but below the temperature at which austenite grain size would become 150.mu. (micron) or higher, hot rolling the steel to obtain total reduction of more than 40% and cooling the hot rolled steel to a temperature within 550.degree.-650.degree. C. at a cooling speed of 5.degree.-20.degree. C./sec.
Briefly speaking, the present invention heats the steel at a lower temperature for rolling and cools the rolled steel to considerably lower temperature range with fairly faster cooling rate.
Differences in these thermal conditions are necessitated in order to obtain superior weldability and good weld zone properties to be obtained by this invention. That is, so as to accomplish such good welding properties carbon content must be kept within a range of 0.005-0.08%, which makes it difficult to obtain both high tensile strength and high toughness by relying on such an extent of controlled rolling followed by accelerated cooling as suggested by said Ouchi, et al. patent.
The present invention has been accomplished by the refinement of austenite grain size brought about by the critical restriction to chemical compositions and rolling conditions combined with lower temperature heating for rolling and cooling down to lower temperature range at a faster cooling rate.